1. Field of the Invention
This invention relates generally to a system and method for testing equipment in a telephone network, and more particularly, to a system and method for remote maintenance and verification of a subscriber loop in a telephone system.
2. Description of Related Art
In the not so distant past, the provider of local phone services not only provided services but also owned and operated the entire network, including the individual subscriber loops and equipment installed in residences and businesses. If a subscriber had a problem, they would contact the provider regardless of the source of the problem. This one-step approach was eliminated with the divestiture of the phone company.
From the phone company's standpoint, determination of whether a problem exists on the network side or the subscriber side is labor-intensive and costly. The problems associated with the determination were mitigated by establishing a demarcation point between the network and the subscriber and installing therebetween a diagnostic device known as a Maintenance Termination Unit (MTU). The MTU is a two-port device which isolates the network and the subscriber loop and has a certain electrical "signature". From the MTU signature, the phone company can estimate whether a resistive or open circuit fault lies within the network or the subscriber loop.
Tests such as the Local Test Desk (LTD) or the Mechanized Loop Tester (MLT) provide means for activating and interpreting the MTU signature to determine whether the problem experienced is the responsibility of the provider or the subscriber.
Reference is made to FIG. 1 which depicts a MTU 8 as applied in a Plain Old Telephone Service (POTS). The MTU 8 comprises a first voltage sensitive switch 12, a second voltage sensitive switch 14, and a DC signature impedance 16. The first voltage sensitive switch 12 interrupts the tip conductor between the network side and the subscriber side while the second voltage sensitive switch 14 interrupts the ring conductor between the network side and the subscriber side. Switches 12 and 14 have a predetermined threshold for which each switch remains open until said threshold is reached. Application of a test voltage greater than the threshold between the tip and ring pair causes switches 12 and 14 to conduct and provide an electrical current path through DC signature impedance 16. The basic test for isolating faults between network and subscriber sides is by measuring the signature between the tip, ring, and ground circuits to identify the faulty circuit. Thus it can be seen that using a DC voltage which ranges above and below the threshold can be used to isolate a fault on either the network or subscriber side of the MTU.
U.S. Pat. No. 4,529,847 issued Jul. 16, 1985 to DeBalko, U.S. Pat. No. 4,396,809 issued Aug. 2, 1983 to Brunssen, and U.S. Pat. No. 4,373,121 issued Feb. 8, 1983 to Sartori et al., disclose some embodiment of the MTU 8 and are herein incorporated by reference. It can be seen that the MTU 8 is well known in the art and can be practiced in various forms as evidenced by the aforementioned patents.
Another problem with the equipment/service dichotomy after the divestiture of the phone company is that equipment evolved with ringers having a substantially different AC impedance than conventional electro-mechanical ringers. The Federal Communications Committee (FCC) which governs communication services and equipment, has set forth under its Rules and Regulations, a preselect value for AC impedance known as a Ringer Equivalence Number (REN). One REN has a certain L-R-C impedance for which the electrical characteristics are known.
Reference is now made to FIG. 2 which depicts a device referred to as a "dummy" or "half ringer" 10. The half ringer 10 is attached across the tip and ring conductors at or near the demarcation point between the network and the subscriber. The half ringer 10 exhibits an impedance of 0.5 REN in accordance with Part 68 of the FCC Rules and Regulations, providing an AC signature useful in performing ringing current tests on the subscriber line.
While these systems and methods for testing AC and DC signatures have been used separately for a number of years, they have never been used collectively. They also suffer a number of problems including the uncertainty in verifying the identity of the subscriber loop under test. Ideally, the central office should be correctly coupled to the tip and ring pair of the subscriber loop under test. However, because of human error or intentional conduct to defraud the service provider of revenue, it is possible that the subscriber loop can be incorrectly wired. Thus, it can be seen that the central office could be under the mistaken belief that a subscriber loop was correctly tested. Therefore, it is important that diagnostic testing be verified in a reliable and an unobtrusive manner to the subscriber. This insures that the central office authenticates its results to protect the integrity of the testing.
It can be seen then that an improved system and method for testing equipment in a telephone network is needed that is reliable and that can be easily adapted to existing systems. It can also be seen that an improved system and method for testing equipment in a telephone network is needed which requires no human intervention for verifying the subscriber loop identity. It can also be seen that integration of both AC and DC signatures in a common unit is both economically and ideologically efficient.